[clang] [Coverage] Introduce `getBranchCounterPair()`. NFC. (PR #112702)

[Jessica Paquette via cfe-commits]

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@@ -1804,9 +1832,10 @@ struct CounterCoverageMappingBuilder
 
     Counter LoopCount =
         addCounters(ParentCount, BackedgeCount, BC.ContinueCount);
-    Counter OutCount =
-        addCounters(BC.BreakCount, subtractCounters(LoopCount, BodyCount));
-    if (OutCount != ParentCount) {
+    auto [ExecCount, ExitCount] = getBranchCounterPair(S, LoopCount);
+    assert(ExecCount.isZero() || ExecCount == BodyCount);
+    Counter OutCount = addCounters(BC.BreakCount, ExitCount);
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ornata wrote:

this looks like it ought to be a helper function as well

https://github.com/llvm/llvm-project/pull/112702